1. Technical Field
This invention relates to a lateral thruster unit externally mounted at the bow, the stern, or the swim platform of a marine vessel for improving the slow speed maneuvering thereof, more particularly to such thruster unit which is capable of remaining in the operating position while the vessel is in motion without incurring a structural damage.
2. Description of the Related Art
The maneuvering of a vessel in close quarters as during docking and undocking is often challenging using only the vessel's main propulsion and steering systems. It is especially difficult for those vessels equipped with an inboard engine with a single screw propulsion system and steered by an ordinary rudder. In most cases, the rudders are configured for optimum performance within the structurally acceptable limits for the maximum operating speed intended for the vessel, and as a result, are often insufficient for adequate steering control at slow speeds. The problem is compounded by the presence of a paddle-wheel phenomenon pronouncedly noticeable with such inboard single screw vessels, wherein the prop-wash created by the rotating propeller generates a swirling movement in the water below the hull to cause the stern of the vessel to drift sideways. In the case of a clockwise rotating propeller as viewed toward the bow, this phenomenon causes the stern of the vessel to tend to drift toward the starboard side while traversing forward and toward the port side while in reverse. The effect is opposite with a counter-clockwise rotating propeller. The steering control at slow speeds may prove further challenging if there is any prevailing wind or current condition which may cause the bow and/or stern of the vessel to veer off from the intended course in an unpredictable manner. As a result of all of these, a vessel with an inboard engine and with single screw propulsion system therefore is known to have a very little steering control while traversing forward at a slow speed and a little or no steering control at all while traversing in reverse.
A vessel with other types of propulsion and steering systems, such as those with an outboard engine or an outdrive system as commonly used in many small modern pleasure and fishing crafts have the ability to adjust the propeller thrust direction via steering control input. That is, by pivoting the rotating axis of the propeller about the vertical axis, a lateral component in the propeller thrustline may be created which will enable the vessel to be steered even at a slow speed and while moving forward or rearward. Also, a vessel with a counter-rotating twin-screw propulsion system as commonly found in many larger pleasure and commercial vessels have the ability to independently control the thrust direction of each of the two propellers. By operating one propeller in the forward direction and the other in reverse, the vessel may be rotated about a vertical axis even without any substantial longitudinal movement of the vessel. Thus, vessels incorporating these types of propulsion systems are inherently more maneuverable. However, piloting even these types of vessels in close quarters, especially the larger of these, many be challenging to all but the most experienced helmsmen, especially with presence of unfavorable wind and/or current conditions.
There are various prior art examples aimed at enhancing the slow speed maneuverability of a vessel, however, the most common commercially available involves an internally mounted bow thruster system. The internal bow thruster systems, as based on such prior arts of Aron, Denston, and Kuss, comprise a thruster unit mounted within a transverse ducting installed at the bow section of a vessel below the waterline connecting the port and starboard sides of the hull. The thruster unit is powered by an electric or a hydraulic motor to drive a single or a multiple propellers coaxially positioned within the tube to induce water flow in the transverse axis of the hull. Such thruster unit may also be mounted within a transverse ducting similarly installed at the stern section of a vessel to create the stern thruster system. The bow or stern thrusters normally remain unused, except only while slow speed maneuvering. The bow and stern thruster systems may be operated independently or simultaneously in opposing directions to cause the vessel to rotate about a vertical axis, or simultaneously in the same direction to cause the vessel to laterally traverse.
There are many problems associated with an internally mounted bow or stern thruster system. First, these are costly to install. The installation requires a major below-the-waterline structural modification to the hull of the vessel to incorporate the ducting, which must be performed while the vessel is dry-docked and by a skilled hand. There are various models of such thruster units currently available in the after-market. However, the complexity and the cost involved with the installation of such are beyond the means of most pleasure vessel owners, and therefore, the use and the benefits of these devices are generally reserved only for the larger and expensive pleasure yachts or commercial vessels.
Second, the installation of the internal thruster unit potentially risks the degradation in the structural integrity in the bow and/or stern sections of the vessel. The weakened structure may develop a failure as a result of navigating through the rough waters or from light bumps or collision with other vessels or the dock structure which otherwise may have been non-detrimental to a hull which had not been so modified.
Third, a structural crack or any failure in the below-the-waterline seal joint, including those seal joints integral to the thruster unit mechanism, will results in the water ingestion aboard the vessel. The leakage aboard may take place while the operator is unaware, and if the rate of such leakage is greater than the rate at which the vessel's overboard pump system is able to discharge overboard the vessel may sink.
Fourth, the ducting will incur marine growth and, unless costly maintenance is performed regularly, result in reduced performance of the thruster system. With the vessel at dock, the cleaning may be performed only by accessing from a duct opening from either side of the hull. The access to clean the interior of the ducting and the gearbox portion of the drive unit within the ducting is difficult, because the accessibility thereto is inhibited by the propeller(s) and the narrowness of the passageway within the ducting. For similar reasons, the maintenance repair is costly. For maintenance operations requiring the removal of the thruster unit from the vessel, either the vessel must first be hauled out of the water and dry-docked, or if the operation must be performed while at dock, special tooling, preparation, and technique will be required to properly seal off all of the openings below the waterline prior to detaching the thruster unit from the ducting.
Fifth, because holes are created on both sides of the bow due to the ducting, the hydrodynamic effects reacting against the discontinuous surfaces on either side of the hull will generate resistance, and thus reduces the operating efficiency of the vessel.
Finally, yet importantly, the internally mounted system detracts from a valuable and limited usable onboard space within the hull.
Some prior art devices, such as of Wardell and Van Breems address some of the concerns dealing with the internally mounted thruster, especially with one regarding the reduced performance due to the hydro-dynamic resistance, by employing a system of mechanisms for retracting and stowing the thruster unit to within the hull cavity while it is being unused. The Van Breems further addresses the maintainability concern by making a detachable thruster unit, which may be easily removed from the vessel from the surface of the deck. However, both of these devices still have the disadvantages of costly hull modification and installation, possible reduction in the structural integrity of the hull, and mechanical complexity resulting from the incorporation of the retracting system.
The prior art of Roestenberg is an externally mounted pivotal bow thruster. This device is advantageous over the others in that it may easily be installed from above the waterline and without a major modification made to the hull of the vessel, and thus it is possible for a moderately skilled user to self-install the device using only ordinary tools. The device also has an advantage of incorporating a pivotal retracting mechanism, which stows the thruster unit out of the water while the vessel is cruising, and therefore preventing any degraded vessel performance due to the hydrodynamic resistance caused by the drive unit. In addition, the maintenance of the device may be performed easily as the entire drive unit and the pivotal actuator assembly may be detached from the vessel. However, the drawback of the Roestenberg is mechanical complexity and delicate nature of the pivotal retracting mechanism, which may easily incur damage while the vessel is navigating through rough waters.
Similarly, the prior art of Pinsof presents an externally mounted stern thruster adapted for mounting on the transom or on the swim platform attached to the transom in a typical small powered vessel. The installation is performed above the waterline and without a major modification performed to the hull of the vessel, and thus, this enables a moderately skilled user to self-install the device using only ordinary tools. However, the Pinsof's stern thruster has a disadvantage of mechanical complexity associated with the linear or rotary deployment and retracting mechanism employed thereby.
While Wardell, Van Breems, Roestenberg, and Pinsof all cite the advantages of incorporating retractability feature to their thruster systems, it is noted that there are also disadvantages associated with the same. The operator must first deploy the thruster unit to the submerged operating position prior to energizing the device. A sustained dry operation of the thruster while the thruster unit is out of the water may lead to premature failures of the drive shaft seal(s) or the motor. In addition, the operator must retract the thruster unit out of the water prior to commencing high-speed cruise, or the hydrodynamic forces may cause damages to the drive unit and its support structure. It is possible to exclusively interlock the deployment and energization of the thruster unit by using position sensors and such, and similarly, to have the retracting operation interlocked with the vessel's instrumentation, so that it automatically retracts when a preprogrammed vessel speed is reached. However, incorporating such safety features will increase complexity and cost and decrease the overall reliability of the thruster system.